Nerve growth factor (NGF) is a protein which has prominent effects on developing sensory and sympathetic neurons of the peripheral nervous system. NGF acts via specific cell surface receptors on responsive neurons to support neuronal survival, promote neurite outgrowth, and enhance neurochemical differentiation. NGF actions are accompanied by alterations in neuronal membranes (Connolly et al, (1981) J Cell Biol 90:176-180; Skaper and Varon (1980) Brain Res 197:379-389), in the state of phosphorylation of neuronal proteins (Yu et al, (1980) J Biol Chem 255:10481-10492; Halegoua and Patrick (1980) Cell 22:571-581), and in the abundance of certain mRNA's and proteins likely to play a role in neuronal differentiation and function (see, for example, Tiercy and Shooter (1986) J Cell Biol 103:2367-2378).
Forebrain cholinergic neurons also respond to NGF and may require NGF for trophic support. Indeed, the distribution and ontogenesis of NGF and its receptor in the central nervous system (CNS) suggest that NGF acts as a target-derived neurotrophic factor for basal forebrain cholinergic neurons (Korschin, (1986) TINS, Nov/Dec, pp 570-573).
Deciphering the causation of human degenerative neurologic disorders has assumed increased importance with the realization that an increasing portion of the population may be affected. Trophic factors, such as NGF, are envisioned to have an important role in the development and maintenance of neurons (Crutcher (1986) CRC Crit Rev Clin Neurobiol 2:297-333), and it has been suggested that deficiencies in the supply or action of trophic factors may contribute to the loss of neurons in a number of neurodegenerative diseases (Appel (1981) Ann Neurol 10:499-505; Hefti and Weiner (1986) ibid 20:275-281).
As stated earlier, cholinergic neurons in the CNS also depend on NGF for survival. One major cholinergic pathway, from the basal forebrain to the hippocampus and neocortex (which responds to NGF), shows early, severe, relatively selective destruction in Alzheimer's disease. It is believed by some workers that NGF may aid in the survival and function of this neuronal population and thus provide a means for therapeutic treatment of Alzheimer's disease.
Amyloid plaques accumulate in the brain in various pathological states: Alzheimer's, Down's syndrome, normal aging, kuru, Creutzfeldt-Jacob disease, Gerstmann-Straussler syndrome, and other neurodegenerative disorders. The extent of amyloid accumulation in Alzheimer's brains correlates with the degree of mental impairment. It is possible that amyloid deposits cause such impairments. The mass of the accumulation itself can physically displace normal brain tissue (Mobley et al, (1983) Soc Neurosci 9:270). In addition, one of the amyloid proteins may promote neurite growth (Yanker et al (1988) Soc Neurosci 14:896; Schubert et al (1988) Science 241:223-226). In some of the above pathological states, massive tangles of neurites are associated with amyloid plaques. Inappropriate neurite growth could alter the maintenance and detour the formation of normal neurite-target connections.
While there has been a proposal that the accumulation of amyloid could be due to excessive amounts of NGF, up until the present invention, there has been no reported demonstration of NGF's involvement in the promotion of amyloid synthesis in animal brain.
It is also known that NGF is capable of encouraging tumor metastasis (Glinsky et al, (1987) Annal New York Acad Sci 496:656-659).
The general concept of using some portion of a protein to block the binding of the entire protein to its intended receptor is known. For example, the peptide Gly-Arg-Gly-Asp-Ser inhibits the colonization of a murine melanoma cell line in lungs of mice into which the melanoma cells were injected (Humphries et al, (1986) Science 255:467-470). This is apparently a result of inhibiting the ability of these cells to bind to the fibronectin, an extracellular matrix which contains this pentapeptide sequence (Akiyama et al, (1985) J Biol Chem 260:10402-10405). Also, the peptide His-Glu-Pro-Pro was able to compete with IgE (of which it is a subsequence) for binding to F.sub.c C receptors in human basophils; this tetrapeptide is thus considered a promising drug for the treatment of allergies (Prenner, (1987) Annal of Allergy 58:332-335).
A subunit of NGF has been proposed by Mercanti et al, (1977) Biochem Biophys Acta 494:412-419 to be an agonist of the full length molecule. However, Romani et al, (1987) Int J Peptide Protein Res 29:99-106; ibid. 107-117, after synthesizing this sequence (designated 10-27/75-88, according to the sequence correspondence in the disulfide linked dimer of identical 118 residue chains), analyzed the activity. The analysis showed that analog activity was not present in the synthetic fragment, nor was this subunit capable of inhibiting binding of NGF, although tested at very high concentration relative to the NGF itself.
Thus, while some peptides of NGF have been synthesized, to date, no one has identified biologically active peptides that retain one or more of the biological activities associated with NGF, nor have any NGF antagonists been identified.